1. Field of the Invention
The present invention generally relates to a planar antenna, and more particularly, to a wide frequency band planar antenna.
2. Description of Related Art
With the advance of wireless internet access technology, a wireless notebook computer allows users to access the internet at a fixed location where an internet station is located, such as, a train station, a university, etc., within a wireless local area network (WLAN). As a result, the wireless notebook has become a mainstream product because it allows the users to freely access the internet. In recent years, WiFi wireless Local Area Network (LAN) has been introduced, which operates at about 2.4 GHz and 5 GHz (these frequencies are referred as a communication carrier frequency modulated by data signals in any modulation technology, such as an orthogonal frequency division multiplex (OFDM) technology). However, the wireless WiFi LAN technology has some drawbacks that limit the use to only the vicinity of the fixed location. These drawbacks refer to a low capacity and a short range (about several hundred meters) for wireless communication carriers, which prevents the users from accessing the internet at any place. Currently, a wireless WiMAX communication technology (i.e. IEEE 820.16 standard) has been developed to overcome the drawbacks of the wireless WiFi LAN technology; that is, WiMAX allows wireless communication carriers to have a higher capacity and a longer communication range without weakening effect such that the internet can be accessed at any place in a metropolitan area where a WiMAX metropolitan area network (MAN) is hosted. In addition, the wireless WiMAX MAN operates at several frequency bands, which have central frequencies at about 2.3 GHZ, 3.4˜3.6 GHz and 5.7˜5.8 GHz, respectively. In response to a need for both WiFi LAN and WiMAX MAN applications, a wide frequency band antenna with its operating frequencies ranging from 2.3 GHz to 5.8 GHz, is needed. This wide frequency band antenna is also referred to as an ultra wide frequency band antenna because of its having a ultra wide range of operating frequencies.
Furthermore, a planar antenna is widely employed in the wireless communication technology because it is easily integrated with a printed circuit board (PCB) and thus provides advantages of compactness and low cost. For example, U.S. Pat. No. 6,535,167 B2 disclosed a laminate pattern antenna capable of operating at a wider frequency band. The laminate pattern antenna comprises an inverted-F-shaped antenna pattern formed as a driven element on the obverse-side surface of a PCB, and an inverted-L-shaped antenna pattern formed as a passive element on the reverse-side surface of the PCB. By setting a path length of the inverted-F-shaped antenna pattern to a specific value, this antenna makes the low-frequency side of its usable frequency range shift to the low-frequency side. Likewise, by setting a path length of the inverted-L-shaped antenna pattern to a specific value, this antenna makes the high-frequency side of its usable frequency range shift to the high-frequency side. As a result, the laminate pattern antenna is able to operate at a wider frequency band; however, its operating frequency is about 2.4 GHz, which limits its application only to WiFi LAN, except for WiMAX MAN. Besides, as the laminate pattern antenna has a complicated structure, its fabricating procedures are accordingly lengthy and the procedures for forming the inverted-F-shaped antenna pattern and then the inverted-L-shaped antenna pattern on both side surfaces of the PCB increases a fabricating cost. Accordingly, the laminate pattern antenna fails to meet a compactness requirement of a planar antenna due to its laminated structure, in addition to its narrow frequency band. Hence, the design of a novel pattern planar antenna that has features of multiple frequency bands, a simple antenna structure and a low fabricating cost is desired.